This invention relates to a process for producing an electroconductive film which is used as films for packaging products of electronic components, in-process dust-proof films or electromagnetic wave-shielding films for electronic equipments. More particularly, this invention relates to a process capable of producing an electroconductive film wherein the resulting film is relatively thin, wherein the film can be continuously produced and wherein the resulting film has a sufficient conductivity and an excellent strength even if the basis weight thereof is low.
It is necessary to protect products such as electronic components such as semiconductor IC and LSI, printed boards and magnetic tapes from the adsorption of dust by virtue of static electricity and troubles by virtue of static electrification, during packaging and delivery. In particular, MOS-type IC and the like tend to occur dielectric breakdown by virtue of static electricity, antistatic treatment is essential. In order to protect products from these static troubles, the products may be packaged with an electroconductive film having a low surface resistivity. Because it is desired that the products such as IC described above can be judged by seeing through the film the packaged contents for the purpose of transactions, it is required that the electroconductive film per se has some transparency when the products are packaged with the electroconductive film.
In producing such an electroconductive film, it is necessary to meet the requirements of conductivity and transparency. In addition to the foregoing, it is necessary that films having a thickness as thin as possible can be obtained in order to retain a more excellent transparency and exhibit a flexibility, and that the sheet-shaped film having a strength sufficient to occur no rupture in process steps can be continuously produced.
Further, in industrial fields of electronic equipments, such as computers and instruments, it is necessary to use an electroconductive film having the functions of static electricity shielding and electromagnetic wave-shielding in an assembly step and the like. While transparency is not necessarily required in these uses, the sufficient conductivity and the product strength are required. Further, it is desired that the sheet-shaped film is continuously produced.
Heretofore, there has been proposed a process for manufacturing an electrocondutive polyolefin material which comprises the steps of incorporating carbon fibers into a pulp of a polyolefin-based material to form a paper stock, and thermally setting the sheet material by heating to a temperature above the melting point of the polyolefin component. (Japanese Patent Publication No. 13214/1977 corresponding to British Pat. No. 1,410,107.)
Further, there has been also proposed a process wherein carbon fibers are replaced by stainless steel fibers. (Japanese Patent Publication No. 41760/1981)
According to our experiments, the following has been found. When an electoconductive film consisting of polyolefinic synthetic pulp and carbon fibers or stainless steel fibers is produced, the polyolefinic synthetic pulp has not substantially physical or chemical bonding properties and therefore the tensile strength, tear strength and surface strength of the resulting sheet material are low. Further, the film can be broken in steps before thermal setting of polyolefin. Thus, it is practically difficult to continuously produce the sheet-shaped film while winding it. Furthermore, it was impossible to produce a product which is a thin film having a small basis weight and which has a sufficient conductivity and a high strength.
In order to compensate the shortage in strength, a single component binder such as hot water-soluble polyvinyl alcohol fibrous binder may be used in combination with the synthetic pulp. However, the melting point of the binder is too low and therefore the molten binder can adhere to the dryer of a paper machine. The foregoing tends to occur the adhesion of tailing onto the sheet, generation of holes and the breakage of paper, and therefore such a process is not preferred.
While the high strength can be obtained by melting the polyolefinic synthetic pulp at a dry part without using any reinforcing agent, many problems take place.
For example, the synthetic pulp exhibits a surface lifting immediately before the pulp melts, and heat melting becomes non-uniform. Further, partial elongation and wrinkle are generated. Eventually, it is impossible to obtain a film having a low basis weight and a high precision.
We have carried out studies in order to overcome the problems of the prior art processes. It has now been found that an electroconductive film having an excellent strength can be continuously produced by incorporating thermoplastic composite fibers described hereinafter into a polyolefinic synthetic pulp to form a paper stock, utilizing the characteristics of such composite fibers and controlling the heating temperature in process steps. We have now found optimum production conditions by means of experiments.